1. Field
The disclosure generally relates to hybrid positioning systems and, more specifically, to methods of integrating a wireless local area network (WLAN)-based positioning system (WLAN PS) and a cellular-based positioning system (CPS) to improve the accuracy of location estimates, increase availability of the positioning service to more users, and also to improve estimation of the expected error in a user's position estimate.
2. Description of Related Art
In recent years the number of mobile computing devices has increased dramatically, creating the need for more advanced mobile and wireless services. Mobile email, walkie-talkie services, multi-player gaming, and call-following are examples of how new applications are emerging for mobile devices. In addition, users are beginning to demand/seek applications that not only utilize their current location but also share that location information with others. Parents wish to keep track of their children, supervisors need to track the locations of the company's delivery vehicles, and a business traveler looks to find the nearest pharmacy to pick up a prescription. All of these examples require an individual to know his own current location or the location of someone else. To date, we all rely on asking for directions, calling someone to ask their whereabouts, or having workers check-in from time to time to report their positions.
Location-based services are an emerging area of mobile applications that leverage the ability of new devices to calculate their current geographic positions and report them to a user or to a service. Examples of these services range from obtaining local weather, traffic updates, and driving directions to child trackers, buddy finders, and urban concierge services. These new location-sensitive devices rely on a variety of technologies that all use the same general concept. By measuring radio signals originating from known reference points, these devices can mathematically calculate the user's position relative to these reference points. Each of these approaches has its strengths and weaknesses depending upon the nature of the signals and measurements and the positioning algorithms employed.
Cellular-based positioning uses cell towers to determine the location of a mobile or user device. Cell towers are identified with a unique identifier in each cellular network in each country. Herein, the unique identifies of cell towers is referred to as cell ID. The cell IDs can be stored in a reference database, accessible by the mobile or user device. In the reference database, the cell ID can be used to link that cell ID to a previously determined location for the cell tower having that cell ID, where the location is also stored in the database. CPS can be based on received signal strength (RSS), time of arrival (ToA), or time difference of arrival (TDoA) from cell towers. Cellular-based positioning systems also can be based on the nearest neighbor technique, in which the entire service area is surveyed and a database of reception characteristics of points in the service area is constructed. The location of the survey points also can be logged, for example, by using a global positioning system (GPS). By comparing the reception characteristics of the mobile device with the surveyed points, the location of the mobile device can be determined. CPS can determine the position of a mobile device through several methods, including received signal strength and time of arrival.
CPS location determinations based on received signal strength use the received power of signals received from cell towers and triangulate the position of the mobile device based upon the received power values. The CPS based on received signal strength is not as accurate a system as compared to WLAN PS. The accuracy of the CPS systems using received signal strength is on the order of hundred meters, while the accuracy of WLAN PS is on the order of tens of meters.
CPS location determinations based on TOA measure the time that cellular radio wave travels to get to the mobile device from the cell tower and calculates the distance from the mobile device to the cell towers based on that time. If travel time of the cellular radio wave is measured correctly, the calculated distance from the cell towers can be accurate as WLAN PS, for example, up to ten meters. However, if the travel time is not measured correctly, the error in the location estimate can be higher than WLAN PS, for example, on the order of hundreds of meters up to a kilometer. The TOA method's accuracy depends on estimating the time of arrival of the cellular radio wave (as the name also indicates). In a normal operational environment of cellular network, the received signal is subject to multipath effect, which means the cellular receiver (of the mobile receiver) receives multiple copies of the transmitted signal reflected from the surroundings of the mobile device. These copies of the transmitted signal can arrive at slightly different times, and the selection of the signal which represents the distance between the mobile device and cell tower (or is the best estimate of distance) can be a challenge and an important part of the design of TOA based CPS. Further, there can be an option to consider all copies of the received signal and calculate distance based on all of them. In this case, there will be a set of distances (between the mobile device and cell tower) which will be passed to a trilateration module in the CPS (discussed herein). The trilateration module considers all the distance measurements from all the cell towers and finds the most likely location of the mobile device.
In the discussion herein, raw CPS measurements from a cell tower are generally intended to mean received signal strength (RSS) and/or times of arrival (TOAs) and/or time differences of arrival (TDOAs). References to cellular data are generally intended to mean the unique address of the cell tower (like cell ID), one or more record(s) of its, one or more power profile(s), and other attributes based on previous measurements of that cell tower. References to a CPS equation are intended to mean a mathematical equation relating the CPS measurements and data to the location of the mobile device.
WLAN-based positioning is a technology which uses WLAN access points to determine the location of mobile users. Metro-wide WLAN-based positioning systems have been explored by a several research labs. The most important research efforts in this area have been conducted by the PlaceLab (www.placelab.com, a project sponsored by Microsoft and Intel); the University of California, San Diego ActiveCampus project (ActiveCampus—Sustaining Educational Communities through Mobile Technology, technical report #CS2002 -0714 ); and the MIT campus-wide location system. There is only one commercial metropolitan WLAN-based positioning system in the market at the time of this writing, and it is referred to herein as the WPS (WiFi positioning system) product of Skyhook Wireless, Inc. (www.skyhookwireless.com).
FIG. 1 depicts a conventional WLAN-based positioning system based on WiFi signals. The positioning system includes positioning software 103 that resides on a mobile or user device 101. Throughout a particular target geographical area, there are a plurality of fixed wireless access points 102 that transmit information using control/common channel signals. The device 101 monitors these transmissions. Each access point contains a unique hardware identifier known as a MAC address. The client positioning software 103 receives transmissions from the 802.11 access points in its range and calculates the geographic location of the computing device using the characteristics of the radio signals. Those characteristics include the MAC addresses, the unique identifiers of the 802.11 access points, the Time of Arrival (TOA) of the signals, and the signal strength at the client device 101. The client software 103 compares the observed 802.11 access points with those in its reference database 104 of access points. This reference database 104 may or may not reside in the device 101. The reference database 104 contains the calculated geographic locations and power profiles of all access points the system has collected. A power profile may be generated from a collection of measurements of the signal power or signal TOA at various locations. Using these known locations or power profiles, the client software 103 calculates the position of the user device 101 relative to the known positions of the access points 102 and determines the device's 101 absolute geographic coordinates in the form of latitude and longitude or latitude, longitude, and altitude. These readings then can be fed to location-based applications such as friend finders, local search web sites, fleet management systems, and an E911 service.
In the discussion herein, raw WLAN measurements from an access point are generally intended to mean received signal strength (RSS) and/or times of arrival (TOAs) and/or time differences of arrival (TDOAs). References to data are generally intended to mean the MAC address, one or more record(s) of it, one or more power profile(s), and other attributes based on previous measurements of that access point. References to a WLAN PS equation are intended to mean a mathematical equation relating the WLAN PS measurements and data to the location of the mobile device.
A WLAN-based positioning systems can be used indoors or outdoors. The only requirement is presence of WLAN access points in the vicinity of the user. The WLAN-based position systems can be leveraged using existing off-the-shelf WLAN cards without any modification other than to employ logic to estimate position.
FIG. 2 illustrates a conventional way of integrating WLAN PS and CPS, which consists of a WLAN PS 201 and a CPS 206 and a location combining logic 210.
WLAN PS 201 and CPS 206 are stand-alone systems, and each can operate independently of the other system. Thus, the result of each system can be calculated independent of the other system. The estimated location along with the expected error estimation of each system can be fed to the location combining logic 210. The expected error estimation is also referred to as HPE (horizontal positioning error) herein. The nominal rate of location update of CPS 206 and WLAN PS 201 is once a second. The location combining logic 210 combines the location estimates calculated in the same second by both systems.
WLAN PS 201 is a conventional system which estimates the location of a mobile device by using WLAN access points (WLAN AP). WLAN PS 201 can include a scanner of WLAN APs 202, a device to select WLAN APs 203, a trilateration module 204, and HPE estimation device 205.
WLAN scanner 202 detects WLAN APs surrounding the mobile device by detecting the received power (RSS, received signal strength) and/or time of arrival (TOA) of the signal. Different methods can be used to detect WLAN APs including active scanning, passive scanning, or combination of passive and active scanning.
The select WLAN APs device 203 selects the best set of WLAN APs to estimate location of the mobile device. For example, if ten WLAN APs are detected and one AP is located in Chicago and the others are located in Boston, without any other information, the Boston APs are selected. This is an indication that Chicago AP has been moved to Boston. In the conventional system, the best set of WLAN APs is selected based on geographical distribution of WLAN APs, in addition to corresponding parameters of WLAN APs, including received signal strength, signal to noise ratio, and the probability of being moved.
Trilateration module 204 uses WLAN APs and corresponding measurements and characteristics to estimate location of the mobile device. Received signal strength or TOA measurements from a WLAN AP are used to estimate distance of the mobile device to the WLAN AP. The aggregation of distance estimates from different WLAN APs with known location is used to calculate location of the mobile device. Trilateration 204 also can use a method which is called nearest neighbor, in which a location with a power profile similar or closest to the power reading of the mobile device is reported as the final location of the mobile device. The power profile of each WLAN AP or entire coverage area can be found in the calibration phase of the system by detailed survey of the coverage area.
HPE estimation device 205 is a module which estimates the expected error of the position estimate of the mobile device. The HPE or Horizontal Positioning Error is calculated based on previously scanned APs and their characteristics and also characteristics of the received signal as it was explained in co-pending Skyhook Wireless application Ser. No. 11/625,450 entitled “System and Method for Estimating Positioning Error Within a WLAN Based Positioning System,” the entire disclosure of which is hereby incorporated by reference.
CPS system 206 can include a cellular scanner 207, trilateration device 208, and the CPS HPE estimation module 209.
The cellular scanner 207 receives signals from one or more cell towers in view of the device, decodes the received signals, and measures received signal strength (RSS) and/or time of arrival (TOA) and/or time difference of arrival (TDOA) of the signals based on the approach taken in the trilateration module 208.
The trilateration device 208 uses measurements from cell towers to estimate the location of the mobile device.
HPE estimation device 209 estimates the expected error of the estimated location. The HPE estimation device 209 is conventional and calculates expected error based on geometry of the cell towers and signal quality of the received signal from cell towers, for example C/N (carrier to noise ratio).
Location combining logic 210 receives simultaneous location estimates and HPE estimates from WLAN PS 201 and CPS 206. Simultaneous location estimations include estimations within one second of each other. The location combining logic 210 reports one estimated location by selecting one WLAN or CPS estimate or by linearly combining them. For example, location combining logic might select the WLAN PS 201 estimate. Otherwise, it may report the CPS estimated location, it might report the final location based on an expected error, or it might report a weighted average of the estimated locations by both systems according to the HPE.